The present invention generally relates to control systems for induction motors, particularly of the sensorless type, and deals more particularly with a control system that provides direct regulation of motor torque and flux.
In recent years, substantial research has been devoted to sensorless drive systems for controlling induction motors in order to achieve high dynamic performance, preserving low cost and endurance. Such systems eliminate the need for position sensors and estimate the positions of the rotor and/or rotor flux. Broadly, these prior systems are either of the type that track machine spatial saliencies, or those that use a variety of control methods for speed and flux control estimation.
The spatial saliencies tracking method requires injection of separate high frequency carrier signal and operates best at low speed. This method is based on the accurate analysis of machine physical structure irregularities and has intrinsic limitations originating from the requirements of separation of fundamental and carrier frequencies and detection of a low amplitude spatial information signal.
The control methods of the second group rely on different control techniques. One of the approaches is based on augmentation of the plant to include dynamics of unknown parameters, and the use of state dependent Riccati equation filter methodology. A particular control method employs two complementary speed and flux observers or estimators. The main speed and flux estimator operates in the monitoring and generating region and requires knowledge of the motor speed sign. The auxiliary estimator operates in generating and braking modes but can be justified if motor speed varies sufficiently slowly. However, the switching between the two estimators can cause divergence when the induction motor is operated at low speed under large load. The high gain speed estimator is utilized in a reference frame related with an estimated flux estimation vector. Rigorous closed loop analysis taking into account rotor resistance variation, and error in estimation of rotor flux and speed provides conditions under which asymptotic torque regulation is achieved.
An object of the invention is to provide a method for controlling an induction motor that does not use a transformation to the rotor flux related frame and that is based on a unique dynamic torque-flux model to control electromagnetic torque and rotor flux.
In accordance with the primary object of the invention, in a preferred embodiment, a method for controlling an induction motor comprises separately tracking the motor""s torque and flux through the use of a dynamic torque-flux model, with stator voltage as an input.
An advantage of the present method is that the control method provides PI regulation of the torque and PD regulation of the flux in a manner that assures asymptotic stability of torque and flux tracking. An added advantage of the invention is that a steady state solution of the torque-flux dynamic model may be used for slow reference signals that minimizes tracking error created due to noise in the current signal.
These and other features and advantages of the present invention may be better understood by considering the following details of a description of a preferred embodiment of the invention. In the course of this description, reference will frequently be made to the attached drawings.